Note: Descriptions are shown in the official language in which they were submitted.
~15333
BIAS DEVICE FOR A COPYING MACHINE
BACKGROUND OF THE INVENTION
This invention relates to a bias device for a copy-
ing machine, which operates to apply a bias voltage to an
electrode which confronts the photo-sensitive member.
FIG. 1 is an explanatory diagram showing the arrange- -
ment of a copylng machine to which the technical concept
of this inventlon i8 applied. Provided around a photo-
sensitive member 1 rotating in the direction of the arrow
A are a discharglng charger 2, a charging charger 3, an
electrode 5 ~or applying a voltage produced by a bias
deYlce 4, and a transferring charger 6. A transferring
sheet B is moved between the photo-sensitive member 1 and
the transferrlng charger 6.
In FIG. 1, reference numeral 7 designates an original
placed on a contact glass plate 8, and reference numeral 9
designates an exposure lamp. Light reflected by the or~gl-
nal 7 is applied through a first mirror 10, a second mirror
11, an in-mirror lens 12, a fourth mirror 13 and a slit 14
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to the photo-sensitive member 1.
One cycle of copying operation consists of the step
of charging by the charging charger 3, the step of expo-
sure by light applied through the slit 14, the step of
development, the step of transferring by the transferring
charger 6, and the step of fixing.
The effect that a bias voltage is applied to the
electrode 5 by the bias device 4, will be described.
The charges left in the photo-sensitive member 1 in
the preceding copying operation are removed by the dis-
charging charger 2, and thereafter new charges are created
in the photo-sensitive member 1 by means of the charging
charger 3. Among these charges, the charges corresponding
to the character region are le~t as they are but the
charges corresponding to the white background region are
removed during the exposure operation in the exposure
~ection. Thus, the character region has a high potent~al,
while the white background region has a low potential,
thus forming a latent image on the photo-sensitive member
1.
In the next process, i.e., in the step of develop-
ment, 8 toner is applied to the latent image. The toner
is stuck to the character region, as a result of which
the latent image is developed. However, in this operation,
the toner i8 stuck to the white background region to an
extent; that is, a portion which should not be developed
i8 al80 developed.
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In order to eliminate this difficulty, during the
development a voltage having the same polarity as that
of the latent image is applied, as a bias voltage, to
the electrode 5, to attract the toner stuck to the whlte
background region, thereby to prevent the adhesion of
the toner to the white background region. In this case,
the amount of toner stuck to the character region is
somewhat reduced, and therefore it is necessary to
minimize this effect by suitably selecting the bias
voltage.
The developed image thus obtained i9 transferred
onto a transferring sheet in the transferring section
provided with the transferring charger 6, and is then
converted into a copy image in the step of ~ixing.
As is apparent from the above description, the bias
voltage applied to the electrode 5 should have a suitable
value in correspondence to the character and white back-
ground regions of an original in any copying process.
In general, a voltage charged in the photo sensitive
member 1 by the charging charger ~ is of the order of
1,000 volts, and the voltages of the portions, correspond-
ing to the character and white background regions, of the
member 1 exposed are of the order of several hundreds of
volts and several tens of volts, respectively. According-
ly, it is suitable that the bias voltage is one hundred
and several tens of volts (slightly higher than the poten-
tial of tbe white background region) to several hundreds
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of volts (slightly lower than the potential of the char-
acter region).
However, there is still a problem. That is, in the
case where the developed image is transferred onto a
number of sheets, the residual potential of the photo-
sensitive member 1 after the transferring cannot be ~plete-
ly eliminated even with the discharging charger 2, and
the residual potential V is increased with the number of
times (n) of repetition as indicated in FIG. 2. (In FIG.
2, reference character V0 designates a residual potential
due to the fatigue of the photo-sens~tive member. After
the copying operation, the residual potential is restored
to the value V0.). Sometimes, the residual potential
reaches several hundreds of volts. In this case, it is
necessary to apply a voltage higher than the residual
potential as the bias voltage to obtain a satisfactory
copy image, because otherwise the portion having the
resldual potential of the previous copying operation
would be developed. Accordingly, the bias voltage applied
should be higher than the residual potential.
From the standpoint of the above description, the
conventional bias device is unsatisfactory. The conven-
tional bias devices are semi-automatic bias devices or
automatic bias devices, none of which are satisfactory
due to the following reasons: In the semi-automatic bias
device, a resistor 19, a diode 20, a lower limit voltage
Vl setting Zener diode 17 and an upper,limit voltage V2
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setting Zener diode 18 are connected to an electric source
16, as shown in FIG. 3, so that when the voltage induced
by the photo-sensitive member is lower than the lower limit
voltage Vl, the lower limit voltage Vl is applied to the
electrode 5, and when the voltage induced by the photo-
sensitive member is higher than the lower limit voltage V
and lower than the upper limit voltage V2, the potential
of the electrode 5 is used as it is, and when the voltage
induced is higher than the upper limit voltage V2, the
upper limit voltage V2 is used as the bias voltage.
When the induced voltage is between the lower limit
voltage Vl and the upper limit voltage V2, the induced
voltage itself is used as the bias voltage, and therefore
it is not the ~uitable voltage which is slightly higher
than the potential of the white background region and the
effect of keeping the background clear is low. This
difficulty may be overcome by increasing the lower limit
voltage ~1 However, in this case, even in the case when
the lnduced voltage is low, the bias voltage is high, and
the character region other than the white background
region is excessively biased, as a result of which the
density is decreased.
On the other hand, in the automatic bias device, as
shown ln FIG. 4, a sensor electrode 21 is provided before
an electrode 5 to which a bias voltage i5 applied, so that
the potential of the photo-sensitive member corresponding
to several tens of millimeters from the end of an original
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(the range in which usually only "a white image" is avail-
able, but no character image is available) is detected by
the sensor electrode 21, and a voltage on which a voltage
Va at the detected potential is superposed by an amplifier
22 is applied through a contact means 23 to the electrode
5. In FIG. 4, a circuit consisting of a transformer 24,
diodes 25 and 26, and capacitors 27 and 28 is to apply a
negative bias ~oltage through the contact means 23 to the
electrode 5 to clean the latter 5.
In the automatic bias device shown in FIG. 4, the
potential of the predetermined range from the end of the
original is detected, and the bias voltage is determined
from the potential thus detected. Therefore, no matter
what ratio of the character region to the white background
region may be in the rear end of the original, the bias
voltage defined by the potential of the front end of the
original is applied to the electrode 5, and the voltage
superposed cannot be changed in that copying cycle.
Therefore, this voltage is not always a bias voltage
suitable for the rear half of the original. mus~ in
the rear half of the original, the white background region
may be developed, or the density of the character region
may be decreased as the case may be.
As is apparent from the above description, any con-
ventional bias devlce suffers from a disadvantage that
contrast between the white background region and the
character region is unstable.
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SUMMARY OF THE INVENTION
Accordingly, an object of this invention is to pro-
vide a bias device for a copying machine, in which all of
the above-described drawbacks accompanying a conventional
bias device have been eliminated.
More specifically, an ob~ect of the invention is to
provide a bias device for a copying machine, in which the
potential of a photo-sensitive member corresponding to an
original is detected at all times to suitably change the
bias voltage, thereby to make the contrast between the
white background region and the character region stable.
The foregoing ob~ect and other objects as well as
the characteristic features of the present invention will
become more apparent from the following detailed descrip-
tion and the appended claims when read in conjunction with
the accompanying drawings.
~RIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is an explanatory diagram showing the arrange-
ment of a copying machine for which a bias device accord-
ing to the invention ls provided;
FIG. 2 is a graphical representation indicating the
variatlon~ in residual voltage of a photo-sensitive member
which are caused as a copying operation iæ repeated;
FIG. 3 i~ a circuit diagram for a description of a
convent1onal semi-automatic bias device;
FIG. 4 is a circuit diagram for a description of a
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conventional automatic bias device;
FIG. 5 is a circuit diagram showing one example of
the bias device according to the invention; and
The parts ~A) through (E) of FIG. 6 are time charts
for a description of the operation of the bias device
according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
A preferred example of a bias device according to
the invention, as shown in FIG. 5, comprises: an on-off
switch SWl for detectingan induced potential, correspond-
ing to a photo-sensitive member, of an electrode 5; a
capacitor 31 for storing the potential of the electrode
5; a reverse-current blocking diode 32; a capacitor dis-
charging transistor 33; an amplifier 34 for superimposing
a voltage on a detection voltage; a change-over switch SW2
for switching a positive bias voltage and a cleaning
negative bias voltage; an on-off switch SW3 for applying
a bias voltage to the electrode 5; a transformer 24;
diodes 25 and 26; and capacitors 27 and 28. The trans-
former 24, diodes 25 and 26 and capacitors 27 and 28 form
a negative bias voltage generating circuit, similarly as
in ~IG. 4.
For the electrode 5, there are provided a cleaning
negative bias period, and an image blas period. In
addition, lt 1s necessary to provide a detection period
in order to detect the potential of the photo-sen~itive
member from one and the same electrode. The detection
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1~5333
period is obtained by switching the above-described
switches SWl through SW3.
The operation of the circuitry shown in FIG. 5 will
be described with reference to the parts (A) through (E)
of FIG. 6, in which reference character Tl designates a
non-~mage period, T2, an image period (being developed),
tl, the negative bias period, and t2, an electrode poten-
tial detection period.
For the period of development (T2), the electrode
potential detecting switch SWl is in off state, the switch
SW3 is in on state, and the armature of the change-over
switch SW2 is tripped to the positive bias contact, so
that the output, or the positive bias voltage, of the
amplifier 34 is applied through the switch SW3 to the
electrode. After the development, i.e., when a non-image
portion occurs on the surface of the photo-sensitive member
(when the period Tl starts), the armature of the switch SW2
is tripped over to the negative bias contact, and the voltage
developed across the capacitors 27 and 28 is applied through
the switches SW2 and SW3 to the electrode 5 to clean the
latter 5. After the cleaning, the switch SWl is turned
on, and the switch SW3 is turned off, as a result of which
the electrode 5 is placed in a floating state that no
voltage is applied to the electrode 5. In this operation, -~
the surface potential of the photo-sen3itive member is
induced and ~t is detected by turning on the switch SWl.
The potential thus detected is applied through the diode
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32 to the capacitor 31 where it is stored. A voltage
obtained by superimposing a certain potential on the
potential thus stored is amplified by the amplifier 34,
whereby the next positive bias voltage is determined.
~hus, when the next Image portion occ~ (the period
T2), the switch SWl is turned off, the switch SW3 is
turned on and the armature of the change-over switch SW2
is tripped to the positive bias contact, then the voltage
thus determined is applied to the electrode 5. The above-
described operation is repeatedly carried out.
When the copying operation is completed, the tran-
sistor ~3 is rendered conductive by a copy completion
signal to remove the charges stored in the capacitor 31.
In the above-described device, no electrode poten-
tial is stored in the capacitor 31 for the copying of the
first ~heet, and therefore a suitable bias voltage must
be applied. However, no problem occurs in practice because
the residual voltage of the surface of the photo-sensitive
member is not so high for the copying of the first sheet.
As is apparent from the above description, the bias
device according to the invention comprises the memory
circuit for storing the potential o~ the surface of the
photo-sensitive member, the circuit for providing the
voltage obtained by superposing the certain potential on
the potential stored in the memory circuit, the switch for
~toring the potential of the non-image interval of the
photo-sensitive member in the memory circuit, and the
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switch for applying the voltage obtained by superposition
to the bias voltage applying electrode. Therefore, even
if the residual voltage of the photo-sensitive member is
increased, the voltage is stored in the memory circuit,
and therefore the suitable bias voltage can be obtained.
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